This invention relates to write current drivers, and particularly to improvements in H-switch circuits used to supply write current to magnetic recording heads.
H-switch drivers are used in magnetic disk drives to supply write current to the magnetic recording head to write data to recording disks. The H-switch is characterized by four transistor switches or the like, each forming a leg of the H, with the write head coil being connected to terminals across the center span of the H. Current is directed in a first direction through the head's coil by operating two semiconductor switches in opposite legs of the H-switch, whereas current is directed in the opposite direction through the head's coil by operating the other two semiconductor switches. There are two primary goals in the design of an H-switch circuit--a fast transition between current directions and fast settling of the write current to its quiescent value after switching. These parameters are affected by two phenomena known as overshoot and undershoot. Overshoot is the amount by which the write current exceeds its quiescent value immediately following a transition. Undershoot is caused by ringing in the write current waveform following a transition, and measures the amount by which the write current falls short of its quiescent value following a transition, as the write current is ringing and settling to its quiescent value.
Write driver circuits have been introduced that attempt to reduce overshoot and/or undershoot by damping the ringing of the write current waveform following a transition in write current direction. For example, one known solution to the ringing problem has been to connect a damping resistor across the terminals of the write head. The resistive damping reduces the settling time for the write current signal flowing through the head. However, resistive damping has several negative effects on the performance of the write circuit, such as increased rise time in the write current transition. Other circuits have also been designed to reduce overshoot and/or undershoot as much as possible. However, there may be some advantages to a certain amount of overshoot designed into the write driver circuit, since higher levels of overshoot typically reduce the rise time of the write current in a transition. At any rate, enhancing the switching characteristics of the H-switch transistors results in improved performance of the write driver circuit, and there is a continuing need in the art for a write driver circuit that achieves this result to improve the overall performance of the circuit.